According to JP-B2-2540738, a Rankine cycle is constructed using components of a refrigerant cycle in a vehicle internal combustion engine or the like. The Rankine cycle is constructed as a generally known waste heat recovery system of a heat generating element such as an internal combustion engine. Waste heat of the heat generating element is recovered, so that the recovered power assists a shaft output of the engine.
FIG. 15 shows a cooling water circuit 20′ according to a related art. Cooling water is circulated between an engine 10 and a radiator 21 in the cooling water circuit 20′. A high-temperature evaporator 210 is connected with the cooling water circuit 20′, so as to form a Rankine cycle for recovering waste heat generated in the engine 10.
The cooling water circuit 20′ includes a switching valve 26 using a thermostat, in general. In this structure, when temperature of cooling water is low, such as in a case where the engine 10 is started, cooling water bypasses the radiator 21 so as not to be introduced into the radiator 21.
In general, a mechanical-type hot water pump 22 is used for circulating cooling water in the cooling water circuit 20′. However, in the related art, an appropriate location of the high-temperature evaporator 210 is not considered in the cooling water circuit 20′. Because the high-temperature evaporator 210 is serially connected with the radiator 21, a flow resistance of water may become large. When the flow resistance becomes large, flow amount of the cooling water decreases. As a result, a cooling performance in the radiator 21 decreases.
The capacity of the hot water pump 22 is in proportion with respect to the rotation speed (revolution) of the engine 10. Accordingly, a flow amount of water circulated by the hot water pump 22 is limited in a low revolution range of the engine 10. Besides, a response of temperature sensitivity of the thermostat used in the switching valve 26 is slow. Therefore, fluctuation of cooling water temperature or fluctuation of cooling water flow amount becomes large depending on a vehicle running condition in a cooling water circuit, which uses the mechanical hot water pump 22 or the switching valve 26 employing the thermostat. Accordingly, a heat amount supplied to the high-temperature evaporator 210 (i.e., Rankine cycle) may become unstable. In addition, when a heat consumption in the Rankine cycle becomes excessively large, the flow amount of cooling water flowing through the cooling water circuit 20′ excessively decreases, and an engine operation (e.g., fuel vaporization in the engine 10) is not effectively performed. Accordingly, the output power of the engine 10 may be reduced, and fuel efficiency may become low.
Further, a compressor is used in the refrigerant cycle, and is also used as an expansion device in the Rankine cycle. Therefore, when the refrigerant cycle is operated in summer or the like, the compressor cannot be used as the expansion device in the Rankine cycle. In this case, a power recovery operation cannot be performed in the Rankine cycle. Accordingly, waste heat of the engine 10 cannot be recovered for assisting the shaft output of the engine 10 or the like.